In motor vehicle technology, a differential pawl is used to improve the drive conditions if at least one driven wheel of the vehicle begins to slip. In this case, the differential pawl substantially synchronizes all the driven wheels, or the driven wheels of one axle, so that they rotate together. Because it acts in an interlocking manner, as a rule, the differential pawl may not be activated during slipping in the engagement direction, to prevent damage to components of the differential pawl and/or damage to the drive mechanism powering the wheels. In addition, the differential pawl as a rule should not remain in the engaged position any longer than absolutely necessary, since the above-mentioned torsional connection of the driven wheels, during normal vehicle operation, can result in damage to the components mentioned above.
Activation devices of the type mentioned above are described in an unpublished German patent application No. P 35 43 894.0. This apparatus of the prior art has actuators which use the pressure of a pressure medium as an energy source. The energy supply is controlled by a control logic or an engagement logic, to which is fed, at a first signal input, an electrical signal produced when at least one drive wheel slips, as a function of the slip, called the wheel slip signal below, and a signal which can be produced arbitrarily is fed to a second signal input. A prerequisite for the action of the activation devices of the prior art is that the driver must produce the arbitrary signal. As a consequence of the input of the arbitrary signal, the control logic assumes a standby position, when the wheel slip signal is received at the input and as long as it remains present thereafter; it also switches from the standby position into an active position, in which it releases energy when the wheel slip signal disappears after synchronization of the driven wheels has been achieved. To disengage the differential pawl, the arbitrary interruption of the arbitrary signal previously generated is necessary.
A disadvantage of these activation devices of the prior art is that they require the intervention of the driver. Therefore, the correct operation of the differential pawl depends on the driver's ability to notice that at least one of the driven wheels is slipping, and to promptly release the differential pawl to protect the above-mentioned components.